JP2004244705A - Nb-CONTAINING CASE-HARDENING STEEL SUPERIOR IN CARBURIZATION PROPERTY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Nb-containing case hardening steel superior in carburization properties. <P>SOLUTION: The Nb-containing case hardening steel superior in carburization properties comprises, by mass%, 0.05-0.35% C, 0.03-1.0% Si, 0.20-2.0% Mn, 0.20-3.5% Cr, 0.01-0.15% Nb, 0.004-0.10% Ti, 0.010-0.10% Al, 0.003-0.05% N, further one or more elements among 1.0% or less Cu, 2.5% or less Ni, 1.0% or less Mo and 0.0003-0.005% B, as needed, so as to satisfy a condition of Ti≥2/5Nb, and the balance Fe with unavoidable impurities. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００２６】
【表２】

【００２７】
表２の結果によると、本発明例は、表面からＣ含有量が０．４１％になる位置までの深さが１．１６〜１．２５ｍｍであり、硬さが５１３ＨＶ以上の有効硬化層深さが１．１８〜１．３５ｍｍであり、結晶粒度が粒度番号７．８〜１０．６であった。
これに対して、Ｎｂを含有しない比較例１は、表面からＣ含有量が０．４１％になる位置までの深さおよび硬さが５１３ＨＶ以上の有効硬化層深さが本発明例と同程度であったが、結晶粒度が粒度番号０で非常に大きかった。
【００２８】
さらに、Ｔｉを含有しないこと以外は成分組成が本発明の範囲内にある比較例２〜４、６および７は、結晶粒度が本発明例と同程度の大きさであったが、本発明例より表面からＣ含有量が０．４１％になる位置までの深さおよび硬さが５１３ＨＶ以上の有効硬化層深さが本発明例より約１０％以上浅くなっていた。
また、Ｔｉを含有しているが、その含有量が２／５Ｎｂ以下である比較例５および８は、結晶粒度が本発明例と同程度の大きさであったが、表面からＣ含有量が０．４１％になる位置までの深さおよび硬さが５１３ＨＶ以上の有効硬化層深さがＴｉを含有しないのと同様に本発明例より約１０％以上浅くなっていた。
【００２９】
次に図１および図２の説明をする。
図１は、Ｃ含有量が０．４１％になる位置までの深さを縦軸にし、Ｔｉ／Ｎｂ含有量を横軸にしたグラフに上記実施例の本発明例および比較例を記入したものである。また図２は、硬さが５１３ＨＶ以上の有効硬化層深さを縦軸にし、Ｔｉ／Ｎｂ含有量を横軸にしたグラフに上記実施例の本発明例および比較例を記入したものである。
Ｔｉ／Ｎｂが２／５以上である本発明例は、Ｃ含有量が０．４１％になる位置までの深さおよび硬さが５１３ＨＶ以上の有効硬化層深さが比較例のＮｂを含有しないものと同程度であったが、比較例のＮｂを含有するものより大幅に深くなっていることが分かる。
【００３０】
【効果】
本発明の浸炭性に優れたＮｂ含有肌焼鋼は、上記構成にしたことにより、結晶粒を粗大化することがなく、焼入れ性がＮｂを含有しない肌焼鋼と同等またはそれ以上になるという優れた効果を奏する。
【図面の簡単な説明】
【図１】Ｃ含有量が０．４１％になる位置までの深さを縦軸にし、Ｔｉ／Ｎｂ含有量を横軸にしたグラフに実施例の結果を記入したグラフである。
【図２】硬さが５１３ＨＶ以上の有効硬化層深さを縦軸にし、Ｔｉ／Ｎｂ含有量を横軸にしたグラフに実施例の結果を記入したグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an Nb-containing case hardening steel having excellent carburizing properties.
[0002]
[Prior art]
Conventionally, power transmission parts such as gears, bearing rolling elements, chains, and spline shafts are required to have high wear resistance, fatigue resistance, pitting resistance, and the like. Therefore, steel for machine structural use, for example, JIS SCM420H (C: 0) .17 to 0.23%, Si: 0.15 to 0.35%, Mn: 0.55 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0. 85 to 1.25% and Mo: 0.15 to 0.35%, the balance being substantially Fe), steel such as SNC415H, SNCM420H, etc., formed by forging, machining or the like, and then carburizing, Manufactured by quenching and tempering.
However, when carburizing steel such as JIS SCM420H or the like, the steel is heated for a long time, so that there is a problem that crystal grains are coarsened and fatigue resistance, pitting resistance and the like are reduced.
[0003]
Therefore, it is known that Al, Ti, Nb, V, and the like are contained in the above steel and the like, and crystal grain growth is suppressed by utilizing precipitation of nitrides and carbonitrides such as AlN, TiN, and Nb (CN). (See Patent Document 1 and Non-Patent Document 1).
Among the elements that suppress the crystal grain growth, Nb has the greatest effect of suppressing the growth of crystal grains, and thus case hardening steel containing Nb has been manufactured.
However, the case hardening steel containing Nb has a problem that the carburizing property is reduced, that is, the carburizing depth per unit time is reduced although the crystal grains rarely grow during carburizing.
[0004]
[Patent Document 1]
JP 2001-20038A [Non-Patent Document 1]
"Metal Handbook" edited by The Japan Institute of Metals, Maruzen Co., Ltd., Showa, December 20, 1982, P803-804
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide an Nb-containing case hardening steel having excellent carburizing properties.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the inventors of the present invention have been intensively studying the cause of the decrease in carburizing property of case hardening steel containing Nb and a method for solving the problem. Is reduced, the inclusion of Ti in the steel containing Nb eliminates the inhibition of C diffusion by Nb, and the amount of Ti that eliminates the inhibition of C diffusion by Nb is Nb × Findings such as 2/5 or more were obtained.
The present invention has been made based on these findings.
[0007]
That is, in the Nb-containing case hardening steel having excellent carburizing properties of the present invention, C: 0.05 to 0.35%, Si: 0.03 to 1.0%, and Mn: 0.20 to 2.0%. , Cr: 0.20 to 3.5%, Al: 0.010 to 0.10%, Nb: 0.01 to 0.15%, Ti: 0.004 to 0.10%, and N: 0.003 -0.05%, and if necessary, Cu: 1.0% or less, Ni: 2.5% or less, Mo: 1.0% or less, and B: 0.0003-0.005% And one or more of the following, and if necessary, S: 0.06% or less, Te: 0.2% or less, Pb: 0.2% or less, Bi: 0.2% or less, Se: One or more of 0.2% or less and Ca: 0.01% or less, and if necessary, Zr: 0.01 to 0.2%, Ta 0.01 to 0.5%, Hf: 0.01 to 0.5%, and V: 0.01 to 0.3%, containing one or more of them, and the condition of Ti ≧ 2 / 5Nb And the balance is Fe and inevitable impurities.
[0008]
[Action]
The carburizing Nb-containing case hardening steel of the present invention does not contain Ti by containing Ti in a range of 0.004 to 0.10% and 2/5 (Nb%) or more. The carburizing depth per unit time becomes deeper than that of the case, which is equivalent to the case hardening steel containing no Nb.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the reason for specifying the component composition in the carburizing steel of the present invention as described above will be described.
C: 0.05-0.35%
C is an element contained for increasing the strength of the core of the steel material and increasing the hardenability. In order to obtain these functions and effects, it is necessary to contain 0.05% or more, preferably 0.15% or more. However, if it exceeds 0.35%, preferably 0.25%, it becomes too hard and the machinability increases. , The content is made 0.05 to 0.35%. The preferred content is 0.15 to 0.25%.
[0010]
Si: 0.03 to 1.0%
Si is a deoxidizing agent at the time of steel smelting and increases the fatigue strength and temper softening resistance of the matrix, and is an element to be contained for them. In order to obtain these effects, it is necessary to contain 0.03% or more. However, if it exceeds 1.0%, it becomes too hard and reduces workability. % to you.
[0011]
Mn: 0.20 to 2.0%
Mn, like Si, is a deoxidizing agent at the time of smelting steel and improves the hardenability, and is an element to be contained for them. In order to obtain these effects, it is necessary to contain 0.20% or more. However, if it exceeds 2.0%, it becomes too hard to lower workability, and the transformation completion time by annealing becomes longer. Content is made 0.20 to 2.0% because it is not economical.
[0012]
Cr: 0.20-3.5%
Cr is an element to be contained for improving hardenability similarly to Mn and for improving the strength of the core by forming a solid solution in the matrix. In order to obtain these functions and effects, it is necessary to contain 0.20% or more, preferably 0.80% or more, but if it is more than 3.5%, preferably more than 3.0%, coarse carbides increase. Since the surface fatigue strength is reduced, the content is set to 0.20 to 3.5%. The preferred content is 0.80 to 3.0%.
[0013]
Al: 0.010% to 0.10% or less Al is an element added as a deoxidizing agent at the time of smelting steel. If the amount remaining in the steel is less than 0.010%, deoxidation of molten steel is not sufficiently performed. , More than 0.10%, the effect is saturated, so the content is made 0.010 to 0.10%.
[0014]
Nb: 0.01 to 0.15%
Nb precipitates carbonitride (Nb (CN)) and suppresses the growth of crystal grains during carburization, and is an element to be contained for that purpose. In order to obtain the effect, the content must be 0.01% or more. However, when the content is more than 0.15%, the effect is saturated. Therefore, the content is set to 0.01 to 0.15%.
[0015]
Ti: 0.004 to 0.10% Ti ≧ 2 / 5Nb
Ti is an element to be contained in the steel containing Nb since the inclusion of Ti in the steel containing Nb eliminates the inhibition of C diffusion by Nb and improves the carburizing property. In order to obtain the effect, it is necessary to contain 0.004% or more and 2 / 5Nb or more as shown in FIGS. 1 and 2. However, if it exceeds 0.10%, the effect is saturated. The content is set to 0.004 to 0.10%.
[0016]
N: 0.003-0.05%
N is a necessary element because N combines with Nb and C to precipitate carbonitride (Nb (CN)) and suppress the growth of crystal grains during carburization or the like. In order to obtain the function and effect, it is necessary to contain 0.003% or more. However, if it exceeds 0.05%, coarse nitrides such as V and Ti are formed and are not dispersed finely. 0.003 to 0.05%.
[0017]
Cu: 1.0% or less, Ni: 2.5% or less, Mo: 1.0% or less Cu, Ni, Mo and B are elements to be contained for improving hardenability. Too much Cu lowers hot workability, while Ni and Mo become too hard, lowering machinability and increasing cost. Therefore, the upper limit is 1.0% for Cu and Mo, and 2.0% for Ni. 5%.
[0018]
B: 0.0003-0.005%
B is an element contained for improving hardenability. In order to obtain the effect, it is necessary to contain 0.0003% or more. However, if it exceeds 0.005%, the hardenability becomes unstable, so the content is made 0.0003 to 0.005%. .
[0019]
Zr: 0.01 to 0.2%, Ta: 0.01 to 0.5%, Hf: 0.01 to 0.5%, V: 0.01 to 0.3%
Zr, Ta, Hf, and V combine with C to form fine precipitates and prevent coarsening of austenite crystal grains, and are thus contained. In order to obtain the effect, it is necessary to contain 0.01% or more, but when Zr exceeds 0.2%, Ta and Hf exceed 0.5%, and when V exceeds 0.3%, the cold workability is reduced. Since the content is reduced, the content is set as described above.
[0020]
S: 0.06% or less, Te: 0.2% or less, Pb: 0.2% or less, Bi: 0.2% or less, Se: 0.2% or less, and Ca: 0.01% or less S, Te , Pb, Bi, Se and Ca are added for improving machinability, but if they are too much, S, Te, Pb, Bi and Se deteriorate hot workability and toughness. , Ca, the effect is saturated, so the upper limit is set as described above.
[0021]
The unavoidable impurities P and O are unavoidable impurities, and it is preferable that the unavoidable impurities reduce the toughness. However, since the cost is increased to reduce the unavoidable impurities, the P is preferably 0.030% or less, and the O is preferably 0.003% or less. .
[0022]
An example of the method of using the carburizing steel of the present invention is to carry out carburizing and diffusion treatment at 850 to 1050 ° C., quench in oil from 830 to 920 ° C. or cool after carburizing and diffusion treatment and then heat to 830 to 920 ° C. And then quenching in oil. Subsequent to quenching, tempering is performed by heating to 150 to 200 ° C. for 1 to 2 hours.
The use of the steel for carburizing of the present invention includes power for gears, bearing rolling elements, drive sprockets, chains, spline shafts, piston pins, cams, rollers, power rollers of toroidal CVT (continuously variable transmission) or input / output disks. Suitable for transmission parts.
[0023]
【Example】
Hereinafter, the present invention will be described with reference to examples.
The steels of the present invention and comparative examples having the component compositions shown in Table 1 below were melted using a 50 kg vacuum induction heating furnace, and cast by an ordinary casting method to obtain ingots. These ingots were hot forged at 1250 ° C. to form a forged material of φ30 mm, and then normalized at 900 ° C. to obtain a test material. Test pieces having a diameter of 25 mm and a length of 130 mm were produced from these test materials. These test pieces were carburized, quenched and tempered in a gas carburizing furnace under the following conditions. The distribution of the C content in the cross section at the center of each of the test pieces subjected to these treatments was measured by a line analysis method using EPMA, and the depth from the surface to the position where the C content became 0.41% was measured. The results are shown in Table 2 below and FIGS. 1 and 2. Further, the hardness distribution of the cross section of the central part of each test piece subjected to the above treatment was measured by the carburizing hardening depth measuring method specified in JIS G 0557, and the depth from the surface to the position where the hardness became 513 HV was measured. (Effective hardened layer depth) was measured. The results are shown in Table 2 below and FIGS. 1 and 2.
[0024]
Carburizing treatment etc. Carburizing diffusion treatment (carburizing 2 hours, diffusion 1 hour) at 1000 ° C for 3 hours, cooling to 850 ° C, holding for 30 minutes, and then quenching in an 80 ° C oil bath. Thereafter, it was kept at 180 ° C. for 60 minutes to perform a tempering treatment. The carburizing gas was mainly composed of RX gas and propane gas, and nitrogen gas was used as a carrier gas.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
According to the results in Table 2, in the example of the present invention, the depth from the surface to the position where the C content becomes 0.41% is 1.16 to 1.25 mm, and the hardness of the effective hardened layer is 513 HV or more. Was 1.18 to 1.35 mm, and the crystal grain size was 7.8 to 10.6.
On the other hand, in Comparative Example 1 containing no Nb, the depth from the surface to the position where the C content becomes 0.41% and the effective hardened layer depth having a hardness of 513 HV or more are comparable to those of the present invention. However, the crystal grain size was very large with a grain size number of 0.
[0028]
Furthermore, Comparative Examples 2 to 4, 6 and 7 in which the component compositions were within the scope of the present invention except that they did not contain Ti had the same crystal grain size as the inventive examples, but the inventive examples The depth from the surface to the position where the C content becomes 0.41% and the effective hardened layer depth with a hardness of 513 HV or more were about 10% or more shallower than the examples of the present invention.
In Comparative Examples 5 and 8, which contain Ti but have a content of 2/5 Nb or less, the crystal grain size is almost the same as that of the present invention, but the C content from the surface is low. The effective hardened layer having a depth and a hardness of 513 HV or more up to a position where the content becomes 0.41% was about 10% or more shallower than the example of the present invention, similarly to the case of not containing Ti.
[0029]
Next, FIG. 1 and FIG. 2 will be described.
FIG. 1 is a graph in which the present invention example and the comparative example of the above embodiment are plotted on a graph in which the vertical axis represents the depth up to the position where the C content becomes 0.41% and the horizontal axis represents the Ti / Nb content. It is. FIG. 2 is a graph in which the present invention example and the comparative example of the above-described example are plotted on a graph in which the vertical axis represents the effective hardened layer depth having a hardness of 513 HV or more and the horizontal axis represents the Ti / Nb content.
In the present invention example in which Ti / Nb is 2/5 or more, the depth to the position where the C content becomes 0.41% and the effective hardened layer depth in which the hardness is 513 HV or more do not contain Nb of the comparative example. Although it was almost the same as that of the comparative example, it was found that the depth was much deeper than that of the comparative example containing Nb.
[0030]
【effect】
The Nb-containing case-hardening steel having excellent carburizing properties of the present invention has the above-described configuration, and does not cause the crystal grains to be coarse, and has a hardenability equal to or higher than that of the case-hardening steel containing no Nb. It has excellent effects.
[Brief description of the drawings]
FIG. 1 is a graph in which the results of the examples are plotted on a graph in which the vertical axis represents the depth to the position where the C content becomes 0.41% and the horizontal axis represents the Ti / Nb content.
FIG. 2 is a graph in which the results of the examples are entered in a graph in which the vertical axis represents the effective hardened layer depth having a hardness of 513 HV or more and the horizontal axis represents the Ti / Nb content.

Claims (4)

In mass% (the same applies hereinafter), C: 0.05 to 0.35%, Si: 0.03 to 1.0%, Mn: 0.20 to 2.0%, Cr: 0.20 to 3.5 %, Al: 0.010 to 0.10%, Nb: 0.01 to 0.15%, Ti: 0.004 to 0.10%, and N: 0.003 to 0.05%. An Nb-containing case-hardening steel excellent in carburization characteristics, which satisfies the condition of ≧ 2 / 5Nb and the balance is Fe and inevitable impurities. One or more of Cu: 1.0% or less, Ni: 2.5% or less, Mo: 1.0% or less, and B: 0.0003 to 0.005% instead of part of the remaining Fe The Nb-containing case hardening steel having excellent carburizing properties according to claim 1, comprising two or more kinds. S: 0.06% or less, Te: 0.2% or less, Pb: 0.2% or less, Bi: 0.2% or less, Se: 0.2% or less, instead of a part of the remaining Fe The Nb-containing case hardening steel having excellent carburizing properties according to claim 1 or 2, wherein one or more of Ca: 0.01% or less are contained. Zr: 0.01 to 0.2%, Ta: 0.01 to 0.5%, Hf: 0.01 to 0.5%, and V: 0.01 to 0 instead of part of the remaining Fe. The Nb-containing case hardening steel having excellent carburizing properties according to any one of claims 1 to 3, wherein the Nb-containing case hardening steel contains one or more of 0.3%.

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